The Multiverse Theory
by HappyEndingsForEveryone
Summary: Sheldon had always believed in the Multiverse theory. It was the most elegant interpretation of quantum mechanics after all. And with his superior intellect, it was only time before he proved it's existence. Now, if only he could find some way to throw it in that none believer Paige's face. TBBT/Young Sheldon Crossover
1. Chapter 1

Disclaimer: I Do Not Own Big Bang Theory

* * *

Sheldon had always believed in the Multiverse theory. It was the most elegant interpretation of quantum mechanics after all.

_Now, if only I had a way to throw my discovery in the face of Paige._ He thought as he stared at his brother Georgies' teenage face in the mirror. In this new universe that he was in, Missy was not his fraternal twin sister. Instead, it was Georgie, who was his twin. Something he had despised from the moment he discovered it.

However, age was where the similarities between them ended. Because no matter the universe, Sheldon Lee Cooper's mind was a thing of beauty. His genius was intact, and so was his past. His early life in Texas was almost identical to the life he lived on Earth-Prime. There were only a few key differences. Instead of going to highschool with Georgie, Sheldon had been forced to attend Middle school with his brother. Luckily he was placed on an advanced curriculum, and his education had not suffered. Well, no more than it had when he attended high school on Earth-Prime.

This change did not shock him. One of the reasons his mother had allowed him to attend high school at the young age of nine was because his older brother would be there with him. Without Georgies' presence in the school, he would have been all alone. True, his father did work there, but George Sr. could not attend classes with him as Georgie could. That meant just as he had audited Professor Sturgis' class; he could only audit classes at the high school once a week.

_No matter._ Sheldon thought, leaving the bathroom. He had graduated high school in less than two years. Then it was off to college for him. Only he doubted that would happen in this universe. Sheldon had never considered how lucky he had been to have an older brother on Earth-Prime. When he was eleven, Georgie was sixteen and capable of driving Missy and himself to school. That small change allowed their mother to not only accompany him to Caltech but three years later to Germany as well. Now, that was no longer possible. Which meant he would be stuck in Texas until his mother deemed him old enough to attend college alone.

"Sheldon, you're going to be late! Hurry up!" his father's voice came from the kitchen. It was the same voice Georgie would have when he was older. And now Sheldon would have it too.

At this age on Earth-Prime, Sheldon would have strode into the kitchen, ignored the looks his siblings directed at his gray plaid pants, and green lantern shirt and explained to his father in detail why they would not be late. He'd mention the speed of his father's truck, the estimated amount of traffic from previous drives, and the most common variable in the equation, his father's driving skills. But now he was older and wiser.

He also missed his dad, and he didn't want to spend their last few years together arguing.

So, he grabbed his messenger bag hanging on his seat at the kitchen table and met his mother at the door.

"I'm ready." He said. "Has the mail come?"

George Copper frowned into his coffee and glanced at his wife. Sheldon had been asking about the mail for going on a week. They were both starting to get worried.

"It's right here, sweetie," Mary said, handing the days mail to their son.

Sheldon tossed the useless letters and ads aside until he found it. In the middle of the stack was his prize. Day after day for months, he had been waiting. Now it was finally here.

His excitement must have been noticeable because his mother smiled and asked. "What's got you so happy?"

He held up his prize, stating what it was with pride. "Our investments have finally paid off."

"Now hold on, son. _Our_ investments?" George asked. "I didn't invest in anything."

"Yes, you did." Sheldon nodded, opening the letter.

"No. I didn't."

"Yes. You did."

Mary set her hand on her husband's arm. His face was growing redder by the second, and she was afraid he was going to burst. She looked at her special little boy and smiled.

"Shelly, why don't you explain what you mean by investments." She said.

"I used the money in our savings account to make us more money," Sheldon explained in the simplest way he could. He loved his mother dearly, his father too, but neither of them were what you would call intellectuals.

"You what!" George exploded.

At the small table in the corner of the kitchen, Missy and Georgie laughed.

"You're in trouble now." Missy giggled.

Sheldon frowned. Why would he be in trouble? "Dad said last year that we needed to cut back on spending."

"Sweetie, that's not something you have to worry about," Mary said with a frown. Maybe it was time to stop letting Sheldon do their taxes?

"And who gave you permission to use that money?" George demanded. "Do you know how hard I work to earn that money."

"I do." Sheldon nodded. "That's why I invested all of it."

"All of-Give me that!" George snatched the letter from Sheldon's hand. He looked down and...damn near had a heart attack. "Hot damn! Mary, look at this!"

"Oh my!"

Sheldon watched silently as his parents passed the check back and forth between them. His father had often complained that the family didn't have enough money growing up. He had now rectified that problem. And he was happy that his parents were happy, but...

"If we don't leave now, then we really will be late for school."

**00**

"Ah, Mr. Cooper!" Principal Petersen greeted enthusiastically as Sheldon walked into his old high school.

Sheldon stopped. That wasn't right. Principal Petersen, like most of his teachers, actively tried to avoid his attention. What could possibly make him want to seek it out today?

Of course, he couldn't ask lest his rudeness be relayed to his mother, so he nodded to his old/current principal.

"Hello, Principal Petersen." He said. "How are you?"

"Pretty good, pretty good. Listen, I've got someone I want you to meet. She's new to the school, and I'd like you to show her around. How about it?"

"Me? Why?" Sheldon asked.

"Well, when you meet someone new for the first time, you put your best foot forward." Principal Petersen said. "And in this school's case, that happens to be you."

"I can't argue with that." Sheldon agreed. He was not only the smartest person_(not just student)_in the school; he also knew the grounds better than anyone having read the blueprints of its construction.

"Great!" Principal Petersen clapped his hands. "Penny, over here!"

_What?_ Sheldon turned his head so fast he was afraid he had gotten whiplash.

"Sheldon, I'd like you to meet our newest student. Penelope Wyatt."

"Hi, Sheldon!" Penny waved, smiling as she bounced on her heels.

"Hello."

Principal Petersen frowned. Sheldon knew why. His greeting was neither warm nor welcoming. In fact, it was just shy of rude. He'd have to remedy before Principal Petersen tattled to his parents.

"I'm Sheldon Cooper. I'll be your guide for the day."

Penny blinked, obviously surprised at his words. "Oh, I thought you were just going to give him a quick walk around school before the first bell."

"That is also acceptable." Sheldon nodded. "Whatever's more comfortable for you."

"That's very sweet of you," Penny said, her smile growing.

"Yes, it was..." Principal Petersen said, watching Sheldon warily. Something was wrong. Sheldon never cared about the comfort of others. And he actively avoided the rest of the student body. Hell, he was surprised the boy hadn't fought him tooth and tail at having to show the new kid around.

Penny reached out and grabbed Sheldon's hand, pulling him towards the front doors of the school. "Come on, Sheldon. You can show me around a bit before we have to go to class."

_Something is very wrong._ Principal Peterson thought as the new girl dragged Sheldon Cooper away. She pulled _SHELDON_ Cooper away. The same boy who didn't hold his own father's hand without mitts. And now he was letting some girl he'd never met before in his life hold his hand. _Girl..hands...oh! No, it couldn't be possible. _

Yet he was seeing it happen right before his eyes. Sheldon was sweet on the new girl.

Wait till George hears this.

**00**

Sheldon glanced at Penny for the fifth time in less than two minutes as they walked the halls of Medford. He couldn't believe she was here. In Galveston. In 1989. At his high school. Logically he knew it was possible. There could any number of universes where Penny attended high school with him. The odds that the first universe he visited would be one of them was-

"Sheldon!" Penny hissed and dragged him into the faculty restroom. "What the hell did you do?"

"Excuse me?" Sheldon asked.

While intellectually, he was an adult, in this universe, he was in the body of a teenager. He could not hope to escape Penny's grasp. Even as a fourteen-year-old, Penny had far more strength than him.

"What the hell did you do?" Penny repeated her question, squeezing his arm tighter. "I went to visit you, and that weird-ass machine you left on the coffee table sucked me into this place."

"Oh, well you see-"

"What the hell even is this place?"

"This is an alternate-"

"God, I could kill you!"

"What did I do?"

Penny gawked at her young neighbor. Why was he so calm? They were trapped. As teenagers. In a world where, for some awful reason, she was fourteen again and living in Texas. And Sheldon had his brother's face.

"If anything, this is your fault," Sheldon said. "My device does not work unless activated. Did you press the activation button?"

"I was curious!" Penny crossed her arm. "I didn't expect to be zapped to another world. How did you even make something like that?"

"Well, it started-"

Penny held up a hand. "Nevermind. Let's skip the science we both know I won't understand. How do we get back?"

"That's easy. I set a timer before entering. At precisely seven thirty-two PM, we will be pulled from this world and back into ours." Sheldon explained. "You understand that you can tell no one of this, correct? Not even Leonard."

"Why not?" Penny asked with a frown. "I figured you'd be shouting a discovery like this from the rooftops the first chance you got."

"Normally, I would. However, I do not believe the world is ready for something of this magnitude." Sheldon said. "What if my device fell into nefarious hands? Or the government confiscated it and killed me to bury the secret of its creation?"

Penny rolled her eyes. "Sheldon, The government isn't going to kill you."

"You better hope not. You're in on this secret too. If they come for me, they're coming for you too."

"Whatever," Penny said, shaking her head. "I'll keep your secret. Just to let you know, I came into your room at seven-thirty. So get ready to be beamed up."

"That's too bad. Time really does fly when you're having fun." Sheldon's shoulders sagged. He was enjoying his time in this world.

"Oh, sweetie." Penny's death grip slackened, and she began to rub his arm, soothing the stinging pain. "Were you having fun with your family?"

"It was nice to see them again. You know the only difference in my family is that Georgie is my twin instead of Missy. Everyone else is exactly the same." Sheldon said.

"So it's kind of like you traveled back in time instead of to a whole new world." Penny nodded. "It's like that for me too — kind of. My brother and sister are my cousins in this world, which is really creepy to think about. But my parents are the same, so there's that. Now, why the hell aren't we being pulled out of here? It's been more than two minutes, dammit!"

Sheldon sighed as Penny paced the small restroom. It was peculiar that his device had not pulled their conscience's from this world.

**00**

**_Apartment 4B,_**

"Oh shit."

Leonard, Howard, and Raj stared down at the shattered remains of whatever Sheldon had been working on.

"You think he'll notice?" Howard asked.

"Of course he is going to notice, dude," Raj answered, pointing at the soda and food on the ground. "We just spilled his dinner all over his new project."

"We can clean that off," Howard said.

Leonard sighed, raising his glasses to rub his eyes. "We can't fix whatever...this thing is. Not before Sheldon gets back."

"Then what are we going to do?" Howard asked. "That thing looked important."

"Ohh, I know." Raj clapped his hands together. "We trash the apartment and tell Sheldon someone stole the whatever thing."

"Great idea, Raj!" Howard smacked his friend on the back. "Let's get to it!"

"No!" Leonard shouted before his friends could start trashing his home. "We are going to wait for Sheldon to get home and calmly explain that this was all a horrible, tragic accident. I'm sure he'll understand."

"Sheldon?" Howard asked, staring at him.

"Understand?" Raj shook his head.

"You're right." Leonard nodded. "Let's start breaking stuff. We don't have much time."

* * *

**Just a fun little story, I couldn't get out of my head!**

**I was rewatching Young Sheldon and found it funny that as a child, Sheldon tried to act and dress like an adult when as an adult, he acted and dressed in a way that most people would call childish. _(I got no problem rocking a superhero Tee.)_ And since I enjoy time travel fic's and this just popped into my head and wouldn't go away, I decided to write it out and post it. **

**Thanks for reading!**


	2. Chapter 2

Disclaimer: I Do Not Own Big Bang Theory

* * *

"George, look at him..."

Mary sighed sadly, nudging her husband with her elbow. In a lunchroom full of highschool students, her little boy was sitting all alone. "It just breaks my heart." She said.

"Poor little guy." George shook his head. "All alone."

"I don't see why his brother can't sit with him," Mary said, looking at her other son. Who was at a table surrounded by friends. There was even an open seat where Sheldon could sit.

"C'mon, Mary." George scoffed and asked. "When you were in high school, would you have lunch with someone like Sheldon?"

Mary glared at her husband. "Yes, I would've."

"Well, there's something wrong with you."

"I'm gonna keep him company." Mary took a step forward. Her little boy wasn't going to be alone, not if she could help it.

"Hey, Mary, you can't." George grabbed her arm, holding her in the doorway. "You gotta think this through. Right now, the other kids are just ignoring Sheldon. What happens if he's sitting with his mommy?"

"They could mistake me for a senior," Mary said, turning to her husband. His skeptical stare made her want to slap him. "Well, I look younger than you."

"Just give this time," George said. "Alright? It'll work itself out."

Mary hoped so. Sheldon had been going to Medford for nearly five years now, and he didn't have a friend in the world. She was starting to worry he never would.

"As a matter of fact, you know what Tom told me happened just this morning?" George asked, a sly proud smirk on his face.

"What happened?" Mary asked warily.

"Sheldon was supposed to show a new student around. This girl Penny, from Nebraska." George pointed at his son. "Sheldon let her hold his hand and drag him around the school. From what I gather, Sheldon didn't complain once."

"What?"

George nodded, chuckling at the look of his surprise on his wife's face. "I was just as shocked at you. Sheldon doesn't let anyone, but you and your mother touch him. Not if he can help it."

Mary turned to watch her son. Just in time to see a pretty blonde girl cut through the cafeteria, heading straight for her Sheldon and take the seat right next to him.

"Is that her?" She asked.

**00**

"You know your parents are watching you, right?" Penny asked as she sat down next to her only friend in the world. And she wasn't just being dramatic. Sheldon was literally her only friend in this new world.

Sheldon nodded and set his PB&J down on the lunch table. "I do. My mother is worried that I do not have any friends. I have not seen my old friend Tam so I will have to find a new friend to ease her worrying."

Penny rolled her eyes, a small smile playing on her lips. Even after all the years together, she still found it funny how someone as smart as Sheldon could be so clueless.

"You already have a friend, sweetie." She said. "And as your friend, I think I have the right to ask. Why the hell are we still here? Did your machine-thingy malfunction?"

Sheldon snorted. "Not likely. I've been using it for months without issue. It's more likely that Leonard, Howard, or Rajeesh broke my device, and we are now stranded in this world indefinitely."

"What?" Penny dropped the fry she was about to eat. "Sheldon, we can't be stuck in this world. I have a husband. You have a wife. We have to get home."

"Correction. You used to have a husband. And I used to have a wife. In this world, we are both unmarried teenagers." Sheldon pointed out.

"Sheldon!"

"Penny, I don't understand your objection to our current predicament," Sheldon said, shaking his head. "While I am sure you miss Leonard, you should, as they say, look on the bright side. From what I have gathered from multiple sources of fiction, most people would be ecstatic when given the chance to relive their life. There must be something in your past that you've always wanted to change."

"Is that what you were doing here?" Penny asked. "Changing the life for the Sheldon in this universe?"

"Yes. After my brother and I reconciled, I felt...bad about my past. That I had taken my life growing up for granted, I couldn't change that on Earth-Prime, so I created an interdimensional device and came here."

"Sheldon, sweetie," Penny patted his hand. "You understand that's crazy, right?"

"I'm not crazy. My-"

"Mother had you tested, I know." Penny cut in. She sighed and glanced at the windows across from their table. "Maybe this won't be so bad."

She was fourteen again. And after meeting the guys, she had come to some hard conclusions about who she was and who she used to be. She was a bully growing up. Sheldon knew. She had told him at his birthday party, and she was sure the other guys knew too. Or at least suspected. The hot blonde was never friends with the nerds. And her ex-boyfriends hadn't been the nicest to them. Not that they were nice to all of them either. So they were kind of bullies too, just in a different way.

"Penny, are you alright?" Sheldon asked.

"Uh? Yeah, I'm okay, sweetie. Just thinking."

"About?"

"About what I'm going to do. Now that I'm a kid again, I've got to figure out what to do with my life." Penny sighed. "And what am I supposed to do about Leonard? Should I wait for him? Are you waiting for Amy?"

"Heavens, no!" Sheldon announced, surprising Penny. "While Amy and I connected on Earth-Prime, there is no guarantee we will connect in this universe. She might not even exist in this universe. The same goes for Leonard."

"So you're saying I shouldn't wait for Leonard."

"No, I'm saying that I'm not waiting for Amy," Sheldon said. "You can do whatever you want."

Penny nodded. "Then I'm not going to wait either. You're right about Leonard and Amy. What if I move to California and Leonard isn't there because he doesn't exist? I can't build my life around a guy. No matter who he is."

"We're in agreement then," Sheldon said.

He picked up his PB&J and opened his mouth to take a bite. In his peripheral, he saw Penny staring at him. He sighed and set his sandwich down again.

"Yes, Penny?"

"Oh, nothing." Penny shook her head and picked up a fry. She glanced at Sheldon again and tossed it back onto her tray. "I'm sorry. It's just; you look so different."

"Of course, I look different. In this world, I'm my brother's twin."

"I know, it's just strange is all," Penny said. "I'm used to the old Sheldon look. This one's new."

Which was a massive understatement. Sheldon had never been bad looking. Tall and lanky, maybe, but he wasn't ugly. Now, he was like a seven on her scale to ten. His curly hair was cut shorter than his brothers but was still longer than old Sheldon's hair. And despite probably never working out a day in his life, he had muscles. He was no football player, but he was a lot bigger than what she imagined a teenage Sheldon would be.

_Huh?_ Penny turned back to her lunch. _Does this make me a creep?_

Mentally she was past thirty, and she was rating teenage boys on their looks. Did it count if she was biologically a teenager herself? It was definitely something she was going to have to think about more.

"I have a lot of things I need to think about." She muttered to herself.

Of course, with his Vulcan like hearing, Sheldon heard her.

"There is quite a bit to think about, isn't there." Sheldon agreed. "For instance, what am I going to do without the internet? And should I send Disney an outline of the Marvel Cinematic Universe so I can watch the movies sooner? And if I do, should I then send scripts for the rest of the movies I want to watch?

"Sheldon..." Penny sighed. "I was thinking about things a little more serious.

"Oh, what could be more serious than the largest connected cinematic universe ever created?"

"I don't know. What am I gonna do for the rest of my life? Should I still move to Pasadena? Try and become an actor again, or go straight for Zangen and become a salesperson?"

Penny dropped her head into her hands. God, there was so much stuff she needed to decide.

"You know, Penny, you don't have to decide what to do right now," Sheldon said. "We still have four years before we're adults again, legally speaking. Why don't you try out for a few school plays?

"That's a good idea," Penny smiled. "Thank you, sweetie."


End file.
